This research is to evaluate and compare the effects of four accepted, but quite different periodontal therapy modalities on human patients. The clinical variables to be evaluated are: (1) periodontal pocket depth, (2) periodontal attachment level, (3) presence of gingival bleeding, (4) presence of supragingival plaque, (5) tooth mobility, (6) molar furcation involvement, and (7) relative proportions of certain subgingival microorganism types determined by dark field microscopy. Each patient, with generalized moderate to severe periodontitis, will be treated with Phase I non-surgical periodontal therapy. The effects of the non-surgical periodontal therapy on the variables will be assessed. During Phase I therapy, each of the patient's four dental quadrants will receive either (1) coronal scaling and polish, (2) deep root planing, (3) modified Widman flap surgery, or (4) flap surgery with ostectomy to achieve positive osseous architecture. Each patient will receive maintenance therapy every three months. Yearly data will be gathered to analyze the clinical variables and compare the efficacy of the four modalities of therapy.